Inductors are used in a wide array of applications such as signal processing, noise filtering, power conversion, electrical transmission systems etc. In order to provide more compact and more efficient inductors, the electrically conducting winding of the inductor may be arranged in a magnetically conducting core, i.e. an inductor core.
Inductor cores may be manufactured by pressing a soft magnetic powder material, e.g. an iron powder. The powder may be put into a cavity wherein the powder may be compacted. In some cases it may be desirable to compress the soft magnetic material powder to a high density in order to e.g. increase the magnetic saturation of the final inductor core etc. During manufacturing, this may be accomplished by increasing the pressure applied by the punches. The maximum possible pressure is limited inter alia by the capacity of the press, the size of the inductor core and the type of powder material which is being compressed.
Inductor cores may be manufactured in a variety of designs. FIGS. 1a and 1b illustrate a prior art inductor core 10. In the prior art, this design is sometimes referred to as a pot core design. The inductor core 10 includes a base core portion 11 from which an outer core portion 12 and an inner core portion 13 extend in an axial direction. The winding (left out for simplicity) may be arranged around the inner core portion 13. The base core portion 11 may include a recess 14 and the outer core portion 12 may include an axially extending slit 15. The purpose of the recess 14 is to accommodate a connection portion of the winding e.g. for connecting the winding to electrical components exterior of the inductor core 10. The purpose of the slit 15 is to provide a lead-through for the connection portion of the winding in the outer core portion 12. By virtue of the recess 14, the connection portion will not occupy any valuable winding space within the inductor core 10 wherein a high winding fill factor may be achieved.
The basic geometry of the inductor core, i.e. without any recess 14 and any slit 15, may be comparably quickly and efficiently manufactured in a single pressing operation. It would be desirable to be able to form also the inductor core 10 in single pressing operation. However, the presence of the recess 14 and the slit 15 complicates the geometry and the structure of the inductor core 10 and affects the manufacturing process. More specifically, the inventors have noticed that the punch responsible for pressing the base core portion 11 and the recess 14 becomes biased during pressing wherein the punch bends through the slit 15 and is pressed against the wall of the die. It has further been noticed that this problem becomes increasingly severe as the pressing force is increased and the size of the inductor core is increased.
One way to avoid this problem is to form the recess 14 in the base core portion 11 and the slit 15 in the outer core portion 12 by a milling process after an inductor core having the above-mentioned basic geometry has been pressed. However, a separate milling process increases the total manufacturing time and also requires additional tools, other than pressing tools, for completing the pot core. Moreover, depending on the geometry of the inductor core and the material choice it may in some cases not be practically possible to mill a recess 14 and a slit 15 to a desired shape.
Another way to avoid this problem is to form the inductor core 10 in a press which, in addition to a first set of punches forming the overall structure of the inductor core 10, also includes an additional punch for forming the recess 14 and the slit 15, which additional punch is independently controllable from the first set of punches. However, this results in a much more complicated and expensive press and tooling.
Thus, there is need in the prior art for an inductor core with a recess and a slit which is more cost-efficient and simpler to manufacture with a high efficiency.